Units of pressure are often somewhat confusing, because of the different systems in use. A Torr (named after Torricelli) is the pressure produced by a column of mercury 1 mm high, so equals 1/760th of an atmosphere. The Pascal is now widely used. This is the S.I. unit, and equals a force of one Newton per square metre (in turn, a Newton is the force required to give a 1 kilogram mass an acceleration of 1 metre per second per second.) The Pascal is quite a small pressure, so we often use KiloPascals (kPa), equal to one thousand Pascals. 101.325 kPa equals one atmosphere. Pounds per square inch (psi) used to be common in the U.K., but has now been supplanted in virtually every country other than the U.S. by the S.I. unit. One atmosphere is approximately 15 psi. The Bar is widely used in industry, and is still often used to specify the pressure in compressed gas cylinders, so many gas regulators are calibrated in Bar. One Bar is 100,000 Pa, and for most practical purposes can be approximated to one atmosphere (more precisely, 1 Bar = 0.9869 atm). http://www.engnetglobal.com/tips/convert.asp?catid=17 contains a simple but effective tool to convert between different units of pressure.